They are steel hollowshank rivets that are mechanically formed
(squeezed) at fabrication - something we can't duplicate. Only retrofit
way is to use solid steel rivets and use a squeeze to install them.
http://yardstore.com/tools/squeezes.htm
I'd leave them installed and use either a portable sandblaster or scaler
to clean the rust and then paint the area with silver spray paint
(spraycan works fine). Cleans the area up well. You can see the end
result in the pictures of my overhauls.
Best of luck,
RJ